High adhesion and generally good performance of polyurethane paint is, to a large degree, dependent on thorough (and timely) surface preparations and application techniques. When the polyurethane paint is to be applied over bare metal surfaces, proper chemical surface treatment must be applied. Dwell time limits after chemical treatment must be observed.
For many planes, there is a requirement that the exterior portion be painted. This is necessary for anti-glare or for a camouflage scheme. The surface of the plane may be a combination of abutting materials such as titanium, stainless steel, aluminum and magnesium. On the F-8 aircraft, there has been a chronic history of paint failure on the titanium hot spot (in and around the exhaust tail cone). The painted surface is constantly taxed due to the heat from the cone with resultant paint peelage. In painting the exterior of an F-8 aircraft, the entire plane is prepared by conventional chromate conversion coatings using an aqueous solution. The plane is rinsed and then dried. Since paint does not adhere well in high humidity areas, the plane must be moved or the area allowed to dry. There is also the masking of certain areas such as the canopy and landing gear. All of this takes time. As long as the plane is painted within 24 hours of its aqueous solution, the aluminum and magnesium areas are receptive to the paint. Titanium and stainless steel, however, are time sensitive and lose their activity or receptivity to organic coatings. Re-treatment of titanium and stainless with water-based solutions is messy and requires a drying step which causes the loss of receptivity to recur.